How to find the neighbor of one cell

[Lance Zhang]

Hello team,

I have one quick question.

May I know how to find the neighbor of a cell?

Here is one part of my code:

----------------------------------------------------------------------------------------------------------------------

for (const auto &cell : solid_3d.get_dof_handler().active_cell_iterators()
{
    const unsigned int cell_index = cell->active_cell_index();

    // Loop over all degrees of freedom on the current cell
    for (unsigned int i = 0; i < dofs_per_cell.dofs_per_cell; ++i)
    {
        // Loop over all neighboring cells
        for (const auto &neighbor : Find_neighbors?)){
     ...

---------------------------------------------------------------------------------------------------------------------

Thanks in advance!

Best regards

Lance  

[Wolfgang Bangerth]

On 11/27/23 14:52, Lance Zhang wrote:
>
> May I know how to find the neighbor of a cell?
>
> Here is one part of my code:
> ----------------------------------------------------------------------------------------------------------------------
> for (const auto &cell : solid_3d.get_dof_handler().active_cell_iterators()
> {
>     const unsigned int cell_index = cell->active_cell_index();
>
>     // Loop over all degrees of freedom on the current cell
>     for (unsigned int i = 0; i < dofs_per_cell.dofs_per_cell; ++i)
>     {
>         // Loop over all neighboring cells
>         for (const auto &neighbor : Find_neighbors?)){

You can write this as
for (unsigned int f=0; f<cell->n_faces(); ++f)
{
DoFHandler::cell_iterator neighbor = cell->neighbor(f);
...
}

Best
W.

[Lance Zhang]

Hello Wolfgang,

thanks for your message.

May I know which method or keywords can be used to stand for the non-existing cell and could I use cell->at_boundary() to check whether the cell exists or not?

I would like to know how we can identify if the object of dealii::IteratorRange<dealii::TriaActiveIterator<dealii::DoFCellAccessor<3, 3, false> > >::IteratorOverIterators does not exist. I have tried ==NULL and nullptr, however, it doe snot work.

There is the code snippet below, please take a look:

 for (const auto &cell : solid_3d.dof_handler_ref.active_cell_iterators())
  {

        if(cell->at_boundary()){
          continue;
          }//to check whether the cell exists

          const unsigned int cell_index = cell->active_cell_index();

          if (visited_cells[cell_index])
             continue; // Skip if the cell has been visited by its all neighbors

      
            // Loop over all degrees of freedom on the current cell

        for (unsigned int i = 0; i < solid_3d.dofs_per_cell; ++i)

      {
     
        // Loop over all neighboring cells

        for (unsigned int f = 0; f < cell->n_faces(); ++f)
        {
            auto neighbor = cell->neighbor(f);//find the neighbor ,but not sure if it exists

            if(neighbor->at_boundary())

            continue;//check if the neighbor of cell exists

            int neighbor_index = neighbor->active_cell_index();//get the global index of the cell if it exists

...

The whole code can be found via the link :https://github.com/Lancelof2019/cook_membrane_snippet/blob/main/code_snippet.cpp

Thanks in advance!

Best regards

Lance

Wolfgang Bangerth <bang...@colostate.edu> 于2023年11月28日周二 01:54写道：

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[Daniel Arndt]

Lance,

You would check for the cell being at a boundary first before asking for its neighbor:

// Loop over all neighboring cells
        for (unsigned int f = 0; f < cell->n_faces(); ++f)
        {

            if (cell->at_boundary(f)) 

              continue;

            auto neighbor = cell->neighbor(f);

            int neighbor_index = neighbor->active_cell_index();//get the global index of the cell if it exists

[...]

Best,

Daniel

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[Lance Zhang]

Hello Daniel,

thanks for your message.

I wrote a statement to check if the cell  exists.

 for (const auto &cell : solid_3d.dof_handler_ref.active_cell_iterators())
  {

//***********************************************************************************************************************************//

        if(cell->at_boundary()){
          continue;
          }//to check whether the cell exists

//**********************filter if the cell is at boundary ,if yes, the cell iterator  will  move forward*************************************//
//*******otherwise if the cell exists,the cell index will be calculated*************************************************************//

          const unsigned int cell_index = cell->active_cell_index();

          if (visited_cells[cell_index])
             continue; // Skip if the cell has been visited by its all neighbors

      
            // Loop over all degrees of freedom on the current cell

        for (unsigned int i = 0; i < solid_3d.dofs_per_cell; ++i)

      {
     
        // Loop over all neighboring cells

        for (unsigned int f = 0; f < cell->n_faces(); ++f)
        {
            auto neighbor = cell->neighbor(f);//find the neighbor ,but not sure if it exists

            if(neighbor->at_boundary())

            continue;//check if the neighbor of cell exists

            int neighbor_index = neighbor->active_cell_index();//get the global index of the cell if it exists

Best regards

Lance

[Wolfgang Bangerth]

On 11/29/23 09:23, Lance Zhang wrote:
>
> //***********************************************************************************************************************************//
>         if(cell->at_boundary()){
>           continue;
>           }//to check whether the cell exists

I don't think this is what you want. Here, you are only asking whether
the cell has *any* faces that are at the boundary. But if you want to
know whether a cell has a neighbor across a certain face, you need to
ask whether the cell is at the boundary *at that particular face*.
Daniel's code does that.

Best
W.